Bonding of materials, in particular composite materials, is widely used in a variety of structural applications. Generally, it is desirable that a strength of the bond is able to be determined.
Conventional bond strength testing includes static proof testing. Static proof testing can be performed to the limit load. Limit load is the maximum load that the structure is expected to see in service. However, the cost of static proof testing is high.
In absence of bond strength testing, validation of bond strength for new components is commonly performed using witness coupons. The witness coupons are samples that are assembled at the same time as bonded structure using the same materials and following as closely as possible the bonding process of the real structure. The witness coupons are then tested to failure to validate the quality of the bonding process. While useful as an indicator, these tested coupons are not a part of the real structure of the new components. Additionally, witness coupons are not available for in-service inspection or repair applications of bond strength testing.
Conventional nondestructive evaluation methods, such as ultrasonics, x-rays, and acoustics, detect the presence of a defect such as a void or unbond within a bonded structure but do not the measure the bond strength thereof.
A laser bond strength testing method may be implemented by fast ablation of the surface of a sacrificial surface layer, such as a strip of polyvinyl chloride tape, by action of the laser on the surface creating heat and a plasma which results in a planar compression wave into a surface of the structure under test and a tension wave upon reflection from an opposing surface of the structure under test. The tension wave interrogates the strength of the bonded structure. The weakest element in the structure fails first, be it in the individual elements bonded together or in the adhesive layer or at the interface between the adhesive and the individual elements.
Laser ablation is well suited for the bond inspection method, creating a sub-microsecond rise time compression/tension wave. Typical laser fluence levels range from 1-50 J/cm2 based on available laser sources and the strength of materials under test. More fluence is required to test heavier gauge materials and this must be deposited into the structure carefully so as to not damage the material. Changing the distribution of laser energy across the surface, by using an annular beam, for example, allows more energy to be deposited without damaging the composite and permits the inspection of heavier gauge material.
Carbon fiber reinforced polymer composites are commonly used in large aircraft structures and are commonly bonded with adhesives. The resulting structures are commonly used to support loads in aircraft and other vehicles, and laser bond strength testing methods have been used to test the strength of the bonds.
Although laser bond strength testing methods are effective, the laser required for laser bond strength testing methods is very expensive, limited in available energy, large in size, requires environmental controls and significant electrical power (>10 kW), and it is difficult to transport the laser energy to the areas where bond testing should be performed.
Therefore, nondestructive bond strength testing for complex structures at low cost is needed. Furthermore, as more automated technology is employed for fabrication of carbon fiber reinforced polymer composite, automated nondestructive bond strength testing is needed.
Accordingly, those skilled in the art continue with research and development in the field of bond strength testing systems and methods.